1. Field of the Invention
This invention relates to the operation of circuitry in response to electrical signals developed by the selection of keys or switches; and particularly to resolving errors in operation resulting from improper key selection as, for example, by the operator of a keyboard printer.
2. Description of the Prior Art
Oftentimes, electronic keyboards generate an undesirable bounce or transient in the electrical signal output due to limitations in key design or improper selection or operation of the keys by an operator. The bounce appears as transient impulses which occur within the key generated electrical signals. A problem arises due to the fact that the electronic implementing circuitry associated with key operation or switching is frequently able to function at much greater speeds than either the operator or the mechanical apparatus. Consequently, when a signal bounce occurs due to improper key operation, the succeeding electronic equipment may respond to both the desirable switching action and the undesirable transient effects. To avoid this erroneous interpretation, it has previously been necessary to resort to expensive and complicated methods for eliminating the bounce in the key originated signals or to unduly limit the speed of response of such successive electronic equipment to such signals.
One of the areas in which this problem oftentimes arises involves keyboard control systems for printers, tape punches, typewriters, etc. In such systems, an operator selects or operates one of a plurality of possible switches or keys by depressing keys on a keyboard. The rate at which successive keys are depressed depends upon the reaction time of both the operator and the apparatus. Tests indicate that the repetitive actuation of the same key with the same finger on standard electric typewriters, for example, involves about 80 milliseconds. On the other hand, an operator may select successively different keys using different fingers within a releatively shorter interval of time, such as 5 or so milliseconds.
To date, potential ambiguity introduced by signal bounce and other short term imperfect switching operations has been eliminated by blanking the signal responsive circuitry for a predetermined time interval following key operation - for example, an interval of the order of 30 or so milliseconds. Thus, any additional transients are assumed to occur before the sensing circuitry is again permitted to function. As explained hereinafter, this technique limits the rapid selection of keys by certain operators. Furthermore, it does not avoid certain problems, such as ambiguity, which arise in the event of flutter upon release of a key or other similar problems.